| Summary: | Background: Bradykinesia in Parkinson's disease is associated with a difficulty in selecting and executing motor actions, likely due to alterations in the functional connectivity of cortico-cortical circuits. Objective/Hypothesis: Our aims were to analyse the functional interplay between the posterior parietal cortex and the ipsilateral primary motor area in Parkinson's disease using bifocal transcranial magnetic stimulation, to evaluate its modulation by dopaminergic treatment and its relationship to a simple choice reaction task. Methods: We studied 12 Parkinson's disease patients with and without dopaminergic treatment and 12 healthy controls. A paired-pulse transcranial magnetic stimulation protocol was applied over the right posterior parietal cortex and the right primary motor area using different conditioning stimulus intensities and interstimulus intervals. Reaction and movement times were studied by a simple choice reaction task. Results: In controls, we observed a significant facilitation of motor evoked potential amplitudes at 4 ms interstimulus interval when conditioning stimulus intensity was set to 90% of resting motor threshold. This functional interaction was not observed in Parkinson's disease patients without dopaminergic treatment and was not restored with treatment. Moreover, correlation analyses revealed that Parkinson's disease patients with less impaired parieto-motor interaction were faster in executing reaching movements in a choice reaction time task, suggesting that the functional parieto-motor impairment described here could be related to bradykinesia observed in Parkinson's disease patients. Conclusions: Parieto-motor functional connectivity is impaired in Parkinson's disease. The reduced efficacy of this connection could be related to presence of bradykinesia previously observed in Parkinson's disease.
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